Electrolytic solution for electric double layer capacitor and electric double layer capacitor

ABSTRACT

An electrolytic solution for an electric double layer capacitor is provided. A spiro compound of formula (1) is provided as an electrolyte in an aprotic solvent. The electric conductivity of the solution at −40° C. is 0.85 mS/cm or higher. 
     
       
         
         
             
             
         
       
     
     This electrolytic solution provides a high dissolution of electrolyte in solvent and exhibits excellent conductivity and electrostatic capacity over a wide range of temperatures Further, the electrolytic solution exhibits high resistance to voltage, even under long-term high voltage load, and exhibits suppressed lowering of electrostatic capacity allowing the solution to excel in long-term stability.

TECHNICAL FIELD

The present invention relates to an electrolytic solution for anelectrical double layer capacitor and an electrical double layercapacitor, and in particular, to an electrolytic solution for anelectrical double layer capacitor excelling in low temperaturecharacteristics and the like and to an electrical double layercapacitor.

BACKGROUND ART

An electrical double layer capacitor is a charge accumulation deviceutilizing an electrical double layer formed in the interface ofpolarizable electrodes and an electrolytic solution.

When the electrolytic solution used in an electrical double layercapacitor possesses a low electric conductivity, the internal resistanceof the electrical double layer capacitor increases thereby decreasingthe voltage of the capacitor during charging and discharging. Therefore,an electrolytic solution used in an electrical double layer capacitor isrequired to possess a high electric conductivity.

Another requirement for an electrolytic solution is the capability ofproviding the capacitor using the electrolytic solution with asufficiently large electrostatic capacity.

These characteristics are required to exhibit a small dependence ontemperature. Specifically, the capacitor is required to maintainexcellent electric conductivity and electrostatic capacity at a lowtemperature.

In addition to the above characteristics, the electrolytic solution isrequired to be durable over a long period of time.

When the electrolyte concentration of the electrolytic solution is low,the internal resistance of the capacitor increases during charging witha large current density due to an insufficient amount of ions.Therefore, the electrolytic solution preferably has an electrolyteconcentration as high as possible.

However, because the electrolytes in the electrolytic solution tend toprecipitate at a low temperature when the electrolyte concentration ofan electrolytic solution is increased, the electrolyte concentration ofthe electrolytic solution must be maintained at a low level. This,however, decreases conductivity and impairs the above charging anddischarging characteristics. In order to solve these problems, crystalprecipitation of the electrolyte at a low temperature must be preventedby increasing the solubility of the electrolyte in the electrolyticsolution. In addition, the electrolyte concentration of the electrolyticsolution must be increased in order to overcome the above problems.

As an electrolytic solution conventionally used in electrical doublelayer capacitors, a solution comprising an electfrolyte such as a linearalkyl quartenary ammonium salt (e.g. tetraethyl ammonium salt) and aquartenary phosphonium salt dissolved in an aprotic solvent such asy-butyrolactone (hereinafter referred to as “GBL”) and propylenecarbonate (hereinafter referred to as “PC”) can be given.

However, since these electrolytes have a low solubility in the organicsolvent of about 0.7-1.5 mol/l, the electrolytic solution has a lowconductivity, possibly giving rise to the previously mentioned problemsof impaired charging and discharging characteristics.

In addition, a capacitor using an electrolytic solution comprisinglinear alkyl quartenary ammonium salt, quartenary phosphonium salt, orthe like tends to have a low electrostatic capacity and a large internalresistance at a low temperature. Specifically, the capacitor alsopossesses a problem in regard to major change in the characteristics dueto a temperature change.

Recently, an electrolytic solution comprising N,N′-dialkyl substitutedimidazolium salt having a high solubility in the above organic solventof about 3 mol/l as an electrolyte has been proposed (for example,Japanese Patent No. 2945890).

However, the electrolytic solution comprising N,N′-dialkyl substitutedimidazolium salt as an electrolyte has a high viscosity and an undulydecreased conductivity at a low temperature as compared with theelectrolytic solution comprising linear alkyl quartenary ammonium saltor quarternary phosphonium salt as an electrolyte. The electrical doublelayer capacitor manufactured using this electrolytic solution exhibits aproblem in long term reliability. The electrostatic capacity is reducedgreatly when the capacitor is subjected to a high voltage load over along period of time.

In view of the above, the object of the present invention is to providean electrolytic solution for an electrical double layer capacitorcapable of dissolving an electrolyte in a high concentration, havingexcellent conductivity and ensuring excellent electrostatic capacity ina wide temperature range from a low temperature to a high temperature,possessing a high withstand voltage, and exhibiting excellent long termreliability with electrostatic capacity decreasing only with difficultyunder a high voltage load for a long period of time. The presentinvention also provides an electrical double layer capacitor using theelectrolytic solution.

DISCLOSURE OF THE INVENTION

As a result of diligent research concerning an electrolytic solution foran electrical double layer capacitor, the present inventor hasdiscovered that an electrolytic solution comprising a specific cyclicammonium salt as an electrolyte possesses high solubility and exhibitsexcellent electric conductivity and electrostatic capacity at a lowtemperature, with least characteristic changes in a wide temperaturerange and excellent long term reliability. This finding has led to thecompletion of the present invention.

Specifically, the present invention provides an electrolytic solutionfor an electrical double layer capacitor comprising a spiro compoundshown by the following formula (1) as an electrolyte dissolved in anaprotic solvent, the electrolytic solution having an electricconductivity of 0.85 mS/cm or more at −40° C.

The present invention also provides an electrolytic solution for anelectrical double layer capacitor to be used in cold regions.

The present invention also provides an electrical double layer capacitorto be used in cold regions manufactured using the above electrolyticsolution for an electrical double layer capacitor.

The present invention further provides a method for lowering thetemperature dependence of the electrostatic capacity and/or internalresistance of an electrical double layer capacitor comprising fillingthe inside of an electrical double layer capacitor with an electrolyticsolution comprising spiro-(1,1)-bipyrrolidinium tetrafluoroborate orpiperidine-1-spiro-1′-pyrrolidinium tetrafluoroborate dissolved in anaprotic solvent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of an example of the structureof the electrical double layer capacitor.

The following is an explanation of the symbols used in the figure.

1 Electrical double layer capacitor

2 First electrode (negative)

3 Secondary electrode (positive)

4 First container body

5 Secondary container body

6 Separator

7 Non-conductive material

BEST MODE FOR CARRYING OUT THE INVENTION

The electrolytic solution for an electrical double layer capacitor ofthe present invention will now be described in detail.

The electrolytic solution for an electrical double layer capacitor ofthe present invention comprises a spiro compound of the followingformula (1) as an electrolyte dissolved in an aprotic solvent,

wherein, m and n individually represent a natural number of 3-7, and X⁻is a counter anion.

Although there are no restrictions to the spiro compound (1) used in thepresent invention, m and n in the formula (1) preferably individuallyrepresent a natural number of 4-6, with 4-5 being particularlypreferable.

There are no restrictions to the counter anion for X⁻ in the spirocompound (1). Examples of the counter anion include a tetrafluoroborateanion, hexafluorophosphate anion, perchloric acid anion,bistrifluoromethanesulfonimide anion, and the like. Of these, atetrafluoroborate anion is particularly preferable.

As particularly preferable specific examples of the spiro compound (1)of the present invention, spiro-(1,1′)-bipyrrolidinium tetrafluoroborate(hereinafter referred to as “SBP—BF₄”) shown by the following formula(2), piperidine-1-spiro-1′-pyrrolidinium tetrafluoroborate (hereinafterreferred to as “PSP—BF₄”) shown by the following formula (3),spiro-(1,1′)-bipiperidinium tetrafluoroborate, and the like can begiven.

Since both the SBP—BF₄ and PSP—BF₄ shown by the above formulas (1) and(2) have a high solubility in the aprotic solvent and precipitatecrystals at a low temperature only with difficulty, a highlyconcentrated electrolytic solution can be obtained. The obtainedelectrolytic solution exhibits excellent electrical conductivity in awide temperature range from a low temperature to a high temperature andthe electrical double layer capacitor obtained using this electrolyticsolution exhibits excellent internal resistance and electrostaticcapacity in a wide temperature range from a low temperature to a hightemperature.

Although there are no restrictions to the method for synthesizing thespiro compound (1) of the present invention, the following method forsynthesizing SBP—BF₄ can be given as a specific example.

First, pyrrolidine is reacted with a butane dihalide as a halogenatingagent to obtain a spiro-(1,1′)bipyrrolidinium halide, which is subjectedto electrodialysis using an ion exchange membrane to obtain aspiro-(1,1′)bipyrrolidinium hydroxide aqueous solution.

Next, the obtained spiro-(1,1′)bipyrrolidinium hydroxide aqueoussolution is neutralized with an equivalent amount of tetrafluoroborate(HBF₄) and dehydrated under reduced pressure to obtain SBP—BF₄.

The following can be given as a method for synthesizing PSP—BF₄.

First, piperidine is reacted with a butane dihalide as a halogenatingagent to obtain a piperidine-1-spiro-1′-pyrrolidinium halide, which issubjected to electrodialysis using an ion exchange membrane to obtain apiperidine-1-spiro-1′-pyrrolidinium hydroxide aqueous solution.

Next, the obtained piperidine-1-spiro-1′-pyrrolidinium hydroxide aqueoussolution is neutralized with an equivalent amount of tetrafluoroborate(HBF₄) and dehydrated under reduced pressure to obtain PSP—BF₄.

There are no restrictions to the aprotic solvent used in the presentinvention as long as it is commonly used for an electrolytic solutionfor an electrical double layer capacitor. Examples of the aproticsolvent include PC, GBL, acetonitrile, dimethyl formamide, sulfolane,and 1,2-dimethoxyethane. Of these, PC and/or GBL are preferable in viewof electrolyte solubility, electric conductivity, temperatureindependency of these characteristics, electrolyte durability, toxicity,and the like.

The electrolytic solution used in the electrical double layer capacitorof the present invention is required to have a conductivity of 0.85mS/cm (0.85 mΩ⁻¹cm⁻¹) or more at −40° C.

Specifically, the electrolytic solution of the present invention, whichcomprises a combination of an electrolyte and an aprotic solventobtained by dissolving an electrolyte in an aprotic solvent, is requiredto have a conductivity of 0.85 mS/cm (0.85 mΩ⁻¹cm⁻¹) or more at −40° C.

The electrolytic solution preferably has a conductivity of 1 mS/cm (1mΩ⁻¹cm⁻¹) or more.

Although there are electrolytes exhibiting electrical conductivitywithin the above range other than the spiro compound (1), none of theseelectrolytes fully satisfy all of the above-mentioned propertyrequirements.

The electrostatic capacity decrease rate of the electrical double layercapacitor obtained using the electrolytic solution of the presentinvention at −20° C. from 20° C. (hereinafter referred to as“electrostatic capacity decrease rate” or “K_(C)”) is preferably 5% orless. The electrolyte and aprotic solvent are appropriately selected tosatisfy this requirement. A particularly preferable K_(C) is 3% or less,with a K_(C) of 2% or less being even more preferable.

The electrostatic capacity decrease rate (K_(C)) of the electrolyticsolution is determined using the following equation (3), wherein C(20°C.) is the electrostatic capacity of the electrical double layercapacitor obtained using the electrolytic solution at 20° C. and C(−20°C.) is the electrostatic capacity at −20° C.K_(C)=100×[C(20° C.)−C(−20° C.)]/C(20° C.)  (3)

It is preferable that the electrical double layer capacitor obtainedusing the electrolytic solution of the present invention has an internalresistance at −20° C. of 5 times or less of the internal resistance ofthe electrical double layer capacitor at 20° C. This ratio of internalresistance is indicated by “K_(R)”.

In the electrolytic solution of this invention, it is preferable toselect a combination of the electrolyte and aprotic solvent to produce asaturated electrolyte concentration of 2 mol/l or more at −40° C. Anelectrolytic solution possessing the above-described excellentcharacteristics can be obtained by appropriately adjusting theconcentration of the electrolyte, when the electrolytic solution is madefrom such a combination of the electrolyte and an aprotic solvent.

If the electrolyte concentration of the electrolytic solution is toolow, the conductivity of the electrolytic solution may be insufficient.If the electrolyte concentration of the electrolytic solution is toohigh, the low temperature performance declines remarkably, and theeconomical efficiency of the solution decreases.

Preferable electrolytic solutions of the present invention will now bedescribed in detail using SBP—BF₄ and PSP—BF₄.

As an example of a preferable electrolytic solution, a solutioncomprising SBP—BF₄ dissolved in an aprotic solvent can be given.Although there are no restrictions to the SBP—BF₄ concentration, aconcentration of 0.5-4 mol/l is preferable, with 1-3.5 mol/l beingparticularly preferable. As a solution having a higher electrolyteconcentration, a solution comprising SBP—BF₄ at a concentration of 2-3.5mol/l is preferable, with 2.3-2.7 mol/l being particularly preferable.

As another example of a preferable electrolytic solution of the presentinvention, a solution comprising PSP—BF₄ dissolved in an aprotic solventcan be given. Although there are no restrictions to the PSP—BF₄concentration, a concentration of 0.5-4 mol/l is preferable, with 0.5-3mol/l being particularly preferable. A PSP—BF₄ concentration of 1-2mol/l is even more preferable.

When necessary, the electrolytic solution of the present invention asdescribed above may comprise additives in addition to the electrolyte.Examples of these additives include phosphorous compounds such asphosphoric acid and phosphate; boric acid compounds such as boric acid,a complex of boric acid and a polysaccharide (mannitol, sorbitol, andthe like), and a complex of boric acid and a polyhydric alcohol(ethylene glycol, glycerol, and the like); and nitro compounds such asp-nitrobenzoic acid and p-nitrophenol.

Although there are no limitations to the amount of the additive, anamount within a range of 0-3 wt % of the total amount of theelectrolytic solution is preferable, with 0.1-1 wt % being particularlypreferable.

The electrical double layer capacitor of the present invention ismanufactured by placing a separator between two capacitor polarizableelectrodes, impregnating the polarizable electrodes with theelectrolytic solution of the present invention as a drive electrolyticsolution, and packing the fabricated body in an exterior case.

Although there are no particular restrictions to the polarizableelectrodes, polarizable electrodes formed from a porous carbon materialsuch as activated carbon powder and carbon fiber; a noble metal oxidematerial such as ruthenium oxide; and a conductive polymeric materialsuch as polypyrrole and polythiophene are preferable, with a porouscarbon material being particularly preferable.

There are no particular limitations to the shape of the electricaldouble layer capacitor using the electrolytic solution of the presentinvention. Examples include a film type, coin type, cylinder type, andbox type.

FIG. 1 can be given as one example of the structure of the electricaldouble layer capacitor, comprising a first electrode and a secondelectrode, each formed from a sheet-type carbon electrode, placed oneither side of a separator, impregnated with an electrolytic solution,and sealed in a first container and a second container which areelectrically disconnected by a non-conductive material.

In the electrical double layer capacitor 1 of FIG. 1, the firstelectrode is represented by 2, the second electrode is represented by 3,the first container is represented by 4, the second container isrepresented by 5, the separator is represented by 6, and thenon-conductive material is represented by 7. In the electrical doublelayer capacitor 1 of FIG. 1, the first electrode 2 is a negativeelectrode 2 and the second electrode 3 is a positive electrode 3.

There are no limitations to the material used for the first container 4and the second container 5 as long as it is a conductive material thatwill not corrode in the presence of the electrolytic solution. Examplesinclude aluminum and stainless steel.

From performance and economical view points, the negative electrode 2and the positive electrode 3 connected electrically to these containersare preferably formed from a carbon material such as activated carbonpowder and carbon fiber formed using a binder. There are no limitationsto the material used for the separator 6 placed between the negativeelectrode 2 and the positive electrode 3 as long as the electrolyticsolution can easily pass through the separator and the material iselectrically and chemically stable. Preferable examples include apolyolefin nonwoven fabric, porous Teflon, and rayon paper.

As an example of a method for manufacturing the electrical double layercapacitor, a method comprising filling the space inside the firstcontainer and the second container with the electrolytic solution of thepresent invention thereby impregnating the negative electrode 2 andpositive electrode 3 with the electrolytic solution, and sealing thefirst container 4 and second container 5 with a non-conductive material7 can be given.

As a preferable method for impregnating the electrodes with theelectrolytic solution of the present invention, a method comprisingvacuum drying each of the materials used in the capacitor with heatingat 120-300° C., injecting the electrolytic solution into the negativeelectrode 2 and positive electrode 3 in a dry argon gas, and aging theelectrodes can be given. Aging is preferably conducted by charging thedevice at a voltage of 2-3 V at room temperature for about 5-100 hours.Finally, defoaming under reduced pressure is preferably conductedthereby completing the electrical double layer capacitor of the presentinvention.

The electrolytic solution for electrical double layer capacitor of thepresent invention does not coagulate at a low temperature and possesseshigh electrical conductivity and electrostatic capacity in a widetemperature range. The electrical double layer capacitor obtained usingthe electrolytic solution has a low dependency on temperature.

Since the electrolytic solution can be used stably at a temperature of−20° C., the electrolytic solution is particularly preferably used in anelectrical double layer capacitor for stable use in cold regions of −20°C. or less.

EXAMPLES

The present invention will be described in more detail by examples,which should not be construed as limiting the present invention.

Preparation Example

<Preparation of Electrolytic Solution for Electrical Double LayerCapacitor>

Electrolytic solutions for electrical double layer capacitor (1)-(7)were prepared by dissolving the ammonium salts shown in Table 1 aselectrolytes in PC at the concentrations shown in Table 1.

The reason for different electrolyte concentrations of the electrolyticsolutions (5)-(7) is that each of the solutions was adjusted in order toobtain the highest electrical conductivity possible so that theelectrolytic solution possessed the best possible characteristics.

TABLE 1 Electrolytic Electrolyte concentration solution No. ElectrolyteSolvent (mol/l) 1 SBP-BF₄ PC 2.50 2 SBP-BF₄ PC 1.50 3 PSP-BF₄ PC 2.50 4PSP-BF₄ PC 1.50 5 TEA-BF₄ PC 0.69 6 TEMA-BF₄ PC 1.80 7 TMI-BF₄ PC 2.50The electrolytes shown in the table are as follows: SBP-BF₄:spiro-(1,1)-bipyrrolidinium tetrafluoroborate PSP-BF₄:piperidine-1-spiro-1′-pyrrolidinium tetrafluoroborate TEA-BF₄:tetraethylammonium tetrafluoroborate TEMA-BF₄: triethlymethylammoniumtetrafluoroborate TMI-BF₄: 1,2,3,4-tetramethylimidazoliumtetrafluoroborate

Measurement Example 1

<Method for Measuring Electric Conductivity>

The electric conductivity of the electrolytic solutions at 30° C. and−40° C. was measured using a CM-20S conductivity meter (manufactured byDKK-TOA Corporation).

The results of the measurement are shown in Table 2. The values shown inthe table are the average values of 15 samples.

As can be seen in Table 2, the electrolytic solutions 1-4 of the presentinvention exhibited a higher electrical conductivity in a widetemperature range than the electrolytic solutions 5 and 6 comprisinglinear alkyl quarternary ammonium salts of a conventional electrolyte.

Measurement Example 2

<Method for Measuring Electrostatic Capacity>

90 wt % of activated carbon powder (particle diameter: 20 μm; specificsurface area: 2000 m²/g) and 10 wt % of polytetrafluoroethylene powderwere kneaded and rolled with a roller to form a sheet having a thicknessof 0.4 mm. Disk-shaped electrodes with a diameter of 13 mm φ werepunched from the sheet.

A polypropylene separator was placed between two of the abovedisk-shaped electrodes, the electrodes were impregnated with apreviously prepared electrolytic solution under vacuum and placed in astainless steel case, and the case was sealed by applying a stainlesssteel cap via a gasket to integrate the case and cap, thereby obtaininga coin type electrical double layer capacitor with a rated voltage of3.3 V.

After charging the obtained electrical double layer capacitor at avoltage of 3.3 V and a temperature of 20° C. for one hour, theelectrostatic capacity was determined from the voltage gradient when thecapacitor was discharged at 1 mA (hereinafter referred to as “C(20°C.)”). A CDT-RD charge-discharge tester (manufactured by Power SystemsCo., Ltd.) was used for the measurement.

The results are shown in Table 2. The values shown in the table are theaverage values of 15 samples.

Measurement Example 3

<Method for Evaluating Long-Term Reliability (Method for MeasuringElectrostatic Capacity Decrease Rate)>

After charging each of the electrical double layer capacitors obtainedin the Measurement Example 2 at a voltage of 3.3 V for 1,000 hours in anoven at 70° C., the electrostatic capacity of the electrical doublelayer capacitors was measured at 20° C. in accordance with the abovemethod for measuring electrostatic capacity (hereinafter referred to as“C_(1000hr)”).

Electrostatic capacity decrease rate (%) (hereinafter referred to as“A”).A=100×(C(20° C.)−C_(1000hr))/C(20° C.)

The electrostatic capacity decrease rate (%) of the electrical doublelayer capacitors was determined by the above formula and the results areshown in Table 2. The values shown in the table are the average valuesof 15 samples.

The long-term reliability was evaluated by means of the electrostaticcapacity decrease rate A.

TABLE 2 Electrical conductivity Electrostatic Long term reliabilityElectrolytic (mS/cm) capacity C(20° C.) Decrease solution No. 30° C.−40° C. C(20° C.) (F) (F) C_(1000 hr) (F) rate A (%) 1 20.41 1.53 1.551.55 1.41 9.0 3 19.11 1.45 1.55 1.55 1.44 7.1 5 11.21 1.31 1.57 1.571.34 14.8 6 16.15 1.45 1.55 1.55 1.30 16.2 7 20.27 1.08 1.58 1.58 0.6857.1

As shown in Table 2, the capacitors manufactured using the electrolyticsolutions 5-7 possessed a low electrostatic capacity after being chargedat a voltage of 3.3 V for 1,000 hours in an oven at 70° C., whichresulted in a large electrostatic capacity decrease rate A. Thecapacitors manufactured using the electrolytic solutions 1 and 3 of thepresent invention, on the other hand, exhibited only a smallelectrostatic capacity decrease rate A, whereby they were determined toexcel in long-term reliability.

Measurement Example 4

<Method for Measuring Low Temperature Characteristics>

Except for measuring at a temperature of −20° C. instead of 20° C., theelectrostatic capacity of the electrical double layer capacitors wasmeasured in the same manner as in Measurement Example 2 (hereinafterreferred to as “C(−20° C.)”).

The electrostatic capacity decrease rate K_(C) in regard to C(20° C.) ofMeasurement Example 2 was determined by the following equation (3).K_(C)=100×[C(20° C.)−C(−20° C.)]/C(20° C.)  (3)

Measurement Example 5

<Method for Measuring Internal Resistance>

The internal resistance of the electrical double layer capacitorsprepared in Measurement Example 2 at 20° C. and −20° C. was measuredusing the CDT-RD charge-discharge tester (manufactured by Power SystemsCo., Ltd.).

The results are shown in Table 3.

TABLE 3 Elec- Electrostatic Electrostatic Internal trolytic capacitycapacity resistance solution (F) decrease (mΩ) K_(R) No. 20° C. −20° C.rate K_(c) (%) 20° C. −20° C. (times) 1 1.55 1.53 1.3 22 101 4.6 3 1.551.52 1.9 27 109 4.0 5 1.57 1.35 14.0 31 161 5.2 6 1.55 1.45 6.5 26 1154.4 7 1.58 1.53 3.2 20 96 4.8

As is clear from Table 3, the electrostatic capacity of the electricaldouble layer capacitors manufactured using the electrolytic solutions 1and 3 of the present invention decreased only slightly, thereby showinga small electrostatic capacity decrease rate KC. The capacitorsmanufactured using the electrolytic solutions 1 and 3 of the presentinvention also possessed only a small internal resistance at −20° C.

The capacitors manufactured using the electrolytic solutions 5-7, on theother hand, showed a large electrostatic capacity decrease rate KC.Also, the capacitors manufactured using the electrolytic solutions 5 and6 possessed a large internal resistance.

Although the capacitor manufactured using the electrolytic solution 7exhibited comparatively good low temperature characteristics among thecapacitors manufactured using the electrolytic solutions 5-7, as shownin Table 3, it exhibited an extremely bad electrostatic capacitydecrease rate A, thereby indicating insufficient long term reliability,as shown in Table 2.

The above results show that the electrolytic solutions 1-4 of thepresent invention display excellent overall characteristics.

INDUSTRIAL APPLICABILITY

The electrolytic solution for the electrical double layer capacitor ofthe present invention comprising a spiro compound shown by the formula(1) as an electrolyte dissolved in an aprotic solvent and having anelectric conductivity of 0.85 mS/cm or more at −40° C. exhibitsexcellent electric conductivity and electrostatic capacity in a widetemperature range from a low temperature to a high temperature and ahigh voltage resistance. The electrolytic solution possesses excellentlong term reliability, with the electrostatic capacity decreasing onlywith difficulty under a high voltage load over a long period of time.Furthermore, due to the excellent low temperature characteristics inparticular, the electrical double layer capacitor manufactured usingthis electrolytic solution can be used in a wide range of industriesfrom miniature electronic instruments to large automobiles.

1. An electrolytic solution for an electrical double layer capacitor comprising a spiro compound of the formula (1) as an electrolyte dissolved in an aprotic solvent and having a conductivity of 0.85mS/cm or more at −40° C,

wherein m and n individually represent a natural number from 3-7 and X- represents a tetrafluoroborate anion.
 2. The electrolytic solution for electrical double layer capacitor according to claim 1, wherein the electrostatic capacity of the electrical double layer capacitor when used at −20° C. decreases by 5% or less from the electrostatic capacity when used at 20° C.
 3. The electrolytic solution for electrical double layer capacitor according to claim 1 or 2, wherein the spiro compound shown by the formula (1) is spiro-(1,1)-bipyrrolidinium tetrafluoroborate or piperidine-1-spiro-1′-pyrrolidinium tetrafluoroborate.
 4. The electrolytic solution for electrical double layer capacitor according to claim 1, wherein the aprotic solvent is propylene carbonate and/or γ-butyrolactone.
 5. The electrolytic solution for an electrical double layer capacitor according to claim 1, comprising a mixture of an electrolyte and an aprotic solvent having a saturated electrolyte concentration of 2 mol/l or more at 40° C.
 6. The electrolytic solution for an electrical double layer capacitor according to claim 1, which can be stably used at −20 C.
 7. The electrolytic solution for an electrical double layer capacitor according to claim 1, wherein the spiro-(1,1)-bipyrrolidinium tetrafluoroborate is dissolved in the aprotic solvent at a concentration in a range of 2-3.5 mol/l.
 8. The electrolytic solution for an electrical double layer capacitor according to claim 1, wherein the piperidine-1-spiro-1′-pyrrolidinium tetrafluoroborate is dissolved in the aprotic solvent at a concentration in a range of 0.5-3 mol/l.
 9. The electrolytic solution for an electrical double layer capacitor according to claim 1 for use in cold regions.
 10. An electrical double layer capacitor manufactured using the electrolytic solution according to claim
 1. 11. The electrical double layer capacitor according to claim 10 comprising a first container, a first electrode electrically connected to the first container, a second container, a second electrode electrically connected to the second container, and a separator separating the first electrode and the second electrode, wherein the space inside the first container and the second container is filled with the electrolytic solution and the first container and the second container are sealed with a nonconductive material which prevents electrical connection between the containers.
 12. The electrical double layer capacitor according to claim 10, manufactured using the electrolytic solution according to any one of claims 1-9 for use in cold regions.
 13. A method for lowering the temperature dependence of the electrostatic capacity and/or internal resistance of an electrical double layer capacitor comprising filling the inside of the electrical double layer capacitor with an electrolytic solution comprising spiro-(1,1)-bipyrrolidinium tetrafluoroborate or piperidine-1-spiro-1′-pyrrolidinium tetrafluoroborate dissolved in an aprotic solvent. 